The present invention relates to the field of assemblies of microelectronic components, and more particularly to multi-chip modules having a high temperature co-fired substrate.
The co-fired substrates are obtained by stacking under pressure insulating layers (generally alumina-based) and conducting layers (generally tungsten-based), and then by firing the whole stack at high temperature (typically 1500.degree. C.). Such substrates are often used to manufacture multi-chip modules. In such application, the co-fired substrates have the following advantages:
the module is readily sealed by means of metallic covers, without requiring weak connections by means of glass bead or the like, since the signals can be exchanged via the edges of the substrate; PA1 a co-fired substrate can readily receive circuits on both faces thereof with interconnections through via holes provided in the substrate; PA1 the heat evolved by the microcircuits is well evacuated towards the edges of the substrate whose different layers act as thermal drains.
A disadvantage of the co-fired substrate multi-chip modules is that they are limited regarding the grouping density and the interconnection density of the microcircuits. They are not suitable when high speed signals are to be exchanged between the circuits, because of the resistivity of tungsten and of the relatively high capacity between the conducting layers. Moreover, the accuracy of the conducting regions provided at the substrate surface for connecting the circuits is not very high, since the important thermal shrinkages that occur when the substrate is fired at high temperature cannot be precisely monitored.
In order to increase the circuit connection density in a multi-chip module having a co-fired substrate, it has been proposed to form thin lines layers on the substrate, with organic dielectric layers (polyimide or derived products) interposed therebetween. Disadvantages of such solution are the unreliability of the connection wire bondings, the humidity outgasing of the organic products employed, and the fact that such organic products do not have a good temperature behaviour, so that it is very difficult to seal the module by bonding a cover on the substrate (the conventional Kovar covers are bonded at a temperature higher than 300.degree. C.).
In the article "A composite multi-layer ceramic substrate for high performance multi-chip packages" (Third IEEE/CHMT IEMTS, Oct. 12-14, 1987, Anaheim, Calif., USA, pages 102-108), A. Dohya et al disclose a multi-chip module having a co-fired substrate which is not a high temperature co-fired substrate, but a glass-ceramic substrate which is sintered at about 900.degree. C. A fine multi-layer wiring part is formed on the multi-layer substrate by alternately fabricating a conducting layer and an insulating layer. The conducting layer is obtained by an electrodeposition technique and fired at 850.degree. C. The insulation layer is made from a photosensitive dielectric paste which is fired at 850.degree. C. The use of photosensitive dielectric pastes is not without disadvatages, and limits the interconnection density. In addition, the low temperature difference between the firing temperatures of the substrate and of the additional layers severely limits the performances of the module. The diffusion of the inks into the substrate, which is not negligible, requires a relatively high separation between the conducting lines, and thus limits the interconnection density.
An object of the present invention is to propose another technique for increasing the connection densities in a multi-chip module having a co-fired substrate, that better meets the requirements of practice.